1. Field of the Invention
The invention relates to an electromagnetic frequency doubler comprising a quantum well semiconductor structure capable of amplifying non-linear optical effects and notably capable of amplifying second-harmonic generation.
2. Discussion of the Background
Frequency doublers can advantageously be used in optical recording and reading systems or any other field in which it is sought to have a power source available at a determined wavelength that cannot be generated by the standard materials used. However, the major problem of frequency doublers lies in the phase-matching that must imperatively be set up between the polarization created by the incident electromagnetic wave and the created harmonic wave. Indeed, the created harmonic wave gets propagated in a material with a wave vector k.sub.2.omega. such that: EQU k.sub.2.omega. 2.pi..pi..sub.2.omega. /.eta..sub.2.omega. =2.times.2.pi..eta..sub.2.omega. /.lambda..sub..omega.
where .eta..sub.2.omega. is the refraction index of the material at the frequency 2.omega., if .omega. is the frequency of the incident electromagnetic wave that gets propagated with a vector k.sub.107 such that: EQU k.sub.107 =2.pi. .eta..sub..omega. /.eta..sub..omega.
.eta..sub..omega. being the refraction index of the material at the frequency .omega..
Generally, .eta..sub..omega. is different from .eta..sub.2.omega. and the phase-shift existing between the polarization created by the incident electromagnetic wave and that created by the harmonic wave drastically restricts any efficiency of conversion of the frequency .omega. into 2.omega.. The term coherence length Lc designates the length of material at the end of which this phase-shift reaches .pi., Lc being therefore defined as follows: EQU Lc(k.sub.2.omega. -2k.sub..omega.)=.pi.
or again: EQU Lc=.eta..sub.107 /4(.lambda..sub.2.omega. -.lambda..sub..omega.).
A known approach by which this problem can be overcome consists in using a periodic disturbance with a pitch--so as to form, for example, an index grating or a second-order non-linear susceptibility grating referenced .chi..sup.(2) responsible for the second-harmonic generation. By making an intelligent choice of .LAMBDA. such that Lc =.LAMBDA./2 it is possible to add up the power emitted at the frequency 2.omega. on the path travelled by the incident electromagnetic wave, as illustrated in FIG. 1 representing the progress of the power created at the frequency 2.omega., as a function of the length 1 travelled in the middle of the doubler. Without grating, we obtain the curve P.sub.2.omega.,1 as a function of 1. By using a grating structure in which there are periodically created domains wherein the coefficient .chi.(2) is not zero, separated from domains in which the coefficient .chi..sup.(2) is zero, the curve P.sub.2.omega.,2 is obtained. Finally, by creating a grating of domains separated from -.chi..sup.(2) domains, the curve P.sub.2.omega.,3 with the best performance is obtained.